A method and a radio network node (800) for enabling a switch between transmission modes for sending downlink signals to a User Equipment, UE. A switching unit (800a) selects a first transmission mode when downlink signal quality in the UE is above a switching threshold (Th), and selects a second transmission mode when the downlink signal quality is below the switching threshold. An obtaining unit (800c) obtains a first performance level when the first transmission mode is used and the downlink signal quality is within a specific vicinity range from the switching threshold, and obtains a second performance level when the second transmission mode is used and the downlink signal quality is within the specific vicinity range. An adjusting unit (800d) then adjusts the switching threshold (Th) based on a deviation between the first performance level and the second performance level. Thereby, the adjusted switching threshold may be more or less optimized and used to achieve the best possible performance under the currently prevailing circumstances regardless of the downlink signal quality. (FIG. 8)